Sealants useful in aerospace and other applications must satisfy demanding mechanical, chemical, and environmental requirements. The sealants can be applied to a variety of surfaces including metal surfaces, primer coatings, intermediate coatings, finished coatings, and aged coatings.
Michael addition curing chemistries are often used in acrylic-based polymer systems and, as disclosed in U.S. Pat. No. 3,138,573, have also been adapted for use in polysulfide compositions. Application of Michael addition curing chemistries to sulfur-containing polymers not only results in cured sealants having faster curing rates and enhanced performance including fuel resistance and thermal resistance, but also provides sealants with improved physical properties such as elongation. The use of Michael addition curing chemistries for sulfur-containing polymer compositions useful in aerospace sealant applications is disclosed in U.S. Application Publication No. 2013/0345371, which is incorporated by reference in its entirety.
The compositions disclosed in U.S. Application Publication No. 2013/0345371 employ one or more base catalysts such as amine catalysts. In the presence of a suitable base such as 1,8-diazabicycloundec-7 -ene (DBU) or 1,4 -diazabicyclo[2.2.2 ]octane (DABCO) or a C6-10 primary amine, the thiol-Michael addition reaction is fast and the cure time is typically less than 2 hours. Without a suitable base catalyst, the Michael addition reaction between, for example, a thiol-terminated polythioether and a Michael acceptor is slow providing a pot life, depending on the temperature, of several days to weeks. However, the physical properties of the cured composition are less than desired for certain applications. The reaction mechanisms for thiol-Michael addition reactions are disclosed by Chan et al., Macromolecules 2010, 43, 6381 -6388.
In practice, the foregoing compositions can be provided as two-part formulations in which the thiol-terminated compound and the Michael acceptor are provided as separate components, with the amine catalyst in one or both components, and the two parts mixed shortly prior to use. For example, if the catalytic amine is a tertiary amine, the amine catalyst may be in one or both components, and if the catalytic amine is a primary or secondary amine, the amine catalyst can only be included in the component containing the thiol-terminated compound. Alternatively, the base catalyst may be provided as a third component, and the component containing the thiol-terminated compound, the component containing the Michael acceptor, and the component containing the base catalyst and the three components combined and mixed shortly before use. However, once the components are mixed, the Michael addition reaction proceeds and, depending at least in part on the temperature and on the type of amine catalyst, the pot life is limited to less than 2 hours. Furthermore, as the composition starts to cure, there is little ability to control the reaction rate to take advantage of the complex chemistries taking place after the sealant is applied to a surface. Amine catalyzed systems such as those disclosed in U.S. Pat. No. 6,172,179 typically cure within 2 hours to 12 hours and although exhibiting acceptable fuel resistance and thermal resistance for many aerospace sealant applications, a longer pot life such as from 24 hours to 72 hours and improved performance of the cured product is desirable.
Compositions having extended pot life and a controlled curing rate can be realized by using a controlled release amine catalyst. In these systems, an amine catalyst such as a strong base or primary amine that produces a fast reaction rate is protected or encapsulated and dispersed in a composition. Upon exposure, for example, to ultraviolet radiation, moisture, or temperature, the catalytic amine is released and catalyzes the Michael addition reaction. In certain embodiments, such systems provide a pot life greater than 2 hours to 12 hours and cure within 24 to 72 hours after the useful working time. Controlled release amine catalysts have been used as described in U.S. Application Publication No. 2013/0345389, which is incorporated by reference in its entirety. Use of controlled release catalysts can provide cure on demand systems. Although the performance of cured sealants prepared using controlled release amine-catalyzed Michael addition curable sulfur-containing polymer compositions is acceptable for many aerospace sealant applications, improved properties such as increased tensile strength is desired.